As various parcels of land are developed to create residential neighborhoods, shopping centers, and the like, pervious land covers like vegetation and undisturbed soil are replaced with impervious land covers, such as pavement and rooftops. These impervious surfaces prevent the natural infiltration of precipitation and result in increased storm water runoff. As storm water runoff flows off of a developed location and ultimately to a stream or other body of water, it can physically cause erosion and move soil particles, as well as dissolve and transport chemicals from the land surface to the body of water.
The U.S. Environmental Protection Agency (EPA) and state departments of natural resources/environmental protection are concerned with nonpoint source (NPS) pollution. NPS pollution is that component of water pollution that comes from the transport of materials on the land surface to a receiving stream via storm water runoff. With NPS pollution, there may be multiple pollutants that come from multiple surface locations under the jurisdiction of multiple landowners. One of these NPS pollutants is sediment. Sediment can be generated from the land surface over which the storm water runoff travels as well as from the bed of the stream channel through which the water is transported. Nonpoint source pollutants are responsible for many stream segments not meeting their water quality criteria.
The EPA now requires post-construction controls under its National Pollutant Discharge Elimination System (NPDES) Phase II storm water requirements. Post-construction controls, as incorporated into municipal ordinances and frequently called best management practices (BMPs), often require the location of buffer strips or stream buffers between the development and the stream in an effort to slow down and allow infiltration of the runoff before it reaches the stream, in addition to allowing sediment to drop out. The mechanism for this reduction in velocity is the hydraulic roughness of the vegetative cover of the buffer as the water travels in the form of overland flow. However, the storm water runoff from a development, even as small as a parking lot, often exits the development at a single concentrated location. There is thus no opportunity to take advantage of the benefits of overland flow and the resulting infiltration.
Stream buffers are among the very popular BMPs. With a stream buffer, a strip of land along a stream is precluded from development. With development, there are two major impacts. The first impact is the generation of additional volume of storm water runoff because of the increase in impervious surfaces. The second impact is the increase in NPS pollutants (e.g., sediment, nutrients, petroleum products, heavy metals) that results from the activities associated with development. The role of stream buffers is to slow the velocity of flow in order to physically allow solid materials to settle out and to increase storm water infiltration. Infiltration into the subsurface promotes “treatment” within the soil matrix. Infiltration results in less water arriving at the stream from the surface and reduces the potential for stream erosion with its many subsequent problems.
The expected performance of a buffer is associated with the expected condition of overland flow—slow, shallow flow that occurs over a large surface area. As overland flow progresses from higher to lower locations, the natural topography helps to concentrate flow into larger and larger channels with subsequent increases in velocity. The result of the concentration is the reduction in sedimentation and infiltration and the production of gullies downstream of any kind of storm water discharge location.
In order to make the requirement for buffers more acceptable, EPA documents tout buffers as linear parks, within the continuum from wooded areas adjacent to a stream through a brushy section, and linking to the grassed areas often associated with developed land. These natural areas could potentially increase property values and make up for the lost development areas. A linear park might be provided with walking paths, or such paths might develop organically from use by landowners and neighbors. Unfortunately, paths would exacerbate the natural process of flow concentration that short circuits the performance of the buffers.
What is needed, then, is a mechanism to return concentrated storm water runoff to the distributed overland flow condition to promote sedimentation and infiltration in compliance with EPA requirements and intentions. Increased sedimentation and infiltration will improve the effectiveness of buffers, even as paths help to make buffers more publically acceptable. Effective storm water management also prevents the destructive and costly impacts of gully and stream bank erosion, both of which can undermine expensive infrastructure. To accomplish this redistribution, a device is needed that introduces enough headloss to slow down the water and then allows it to spread out.
Current practice for managing concentrated flow, as from a parking lot, is to line the initial portion of the receiving vegetated channels with riprap or related products. The roughness of the riprap induces headlosses and armors the drainage channel to reduce erosion as the water travels to a stream, but does not serve to promote infiltration or sediment settling. Much opportunity remains for storm water management devices in order to service these regulatory drivers, even as the requirements become more stringent and more broadly applicable.